Coarse aggregate is an important proportion of concrete material, and the change of its size will affect the contribution of aggregate interlock behaviour in concrete beams to shear capacity. In order to consider the effect of the coarse aggregate size on the shear failure of Basalt Fiber Reinforced Polymer (BFRP) beams without stirrup systematically, the meso-scale simulation was selected and a numerical model of BFRP reinforced concrete beams without stirrup was established. The effects of member size and coarse aggregate size on the shear failure mode and shear strength of BFRP reinforced concrete beams were simulated and analyzed. The results show that the size effect exists in the shear strength of BFRP reinforced concrete beams without stirrup. Meanwhile, the increase of the maximum aggregate size improves the shear capacity of concrete beam, and weakens the size effect of shear strength. According to the influence mechanism and law of maximum coarse aggregate size, theoretical formula of size effect on the shear strength of BFRP reinforced concrete beams was established on the basis of Fracture-SEL, and its accuracy and rationality are verified via experimental data.